The term “leadframe” denotes an array of metallic lead elements for a semiconductor component; such arrays are stamped or etched out of sheet metal and are used during the manufacturing process. Initially, the individual lead elements remain interconnected in an outer region of the leadframe and are thereby held in position relative to one another. One or more semiconductor chips are then disposed on designated lead elements of the leadframe and are contacted with other lead elements via so-called bonding wires. Only after the semiconductor chips and parts of the leadframe have been enveloped in a housing made of a compression- or injection-molded plastic compound is the component, with its lead tabs, separated from the leadframe in a desired length. In addition to the lead elements, leadframes often further comprise retention tabs, which are also overmolded by the housing and serve to fix the component in retaining devices during the manufacturing process. If the retention tabs are inserted in the housing to only a very shallow depth, they can easily be pulled out of the housing at the end of the manufacturing process. Failing this, it is possible, but more onerous, to sever the retention tabs close to the housing. To be able to implement the retaining devices as simply as possible, it is desirable for the portions of the lead elements that protrude from, the housing—which portions will be referred to hereinafter as lead tabs—and the retention tabs to lie in one plane. In addition, in order to simplify and automate the process of manufacturing semiconductor components, leadframes usually are not fabricated for a single component, but are provided in the form of endless strips for a large number of identical components. It is also known to mold portions of the housing onto the leadframe even before the semiconductor chip is mounted on it.
The concept of retention tabs can be especially problematic where very small housing dimensions are concerned. One example of a type of component that is provided with such a housing is surface-mounted, light-emitting semiconductor components (LEDs or light-emitting diodes). The housings of such components are typically oblong and have two lead tabs that protrude—axially, if possible—from the housing, which is only negligibly wider than the lead tabs. For the retention tabs to be such that they can be removed merely by pulling them out of the housing, they must already be separated from other lead elements in the region of the housing. In the prior art, this inevitably entails a punched or etched gap between the ends of the respective retention tabs and the adjacent regions of the lead elements.
FIG. 1 exemplarily illustrates this problem associated with leadframes according to the prior art.
FIG. 1 illustrates the central, inner portion of a metallic leadframe 1 according to the prior art in a schematic sectional diagram. The leadframe 1 comprises two electrical lead elements 2, whose outer portions are denominated as lead tabs 3. Further provided are four retention tabs 4, which are separated from the lead elements 2 by punched or etched gaps 5. Further provided is a housing body 6, which is molded onto the leadframe 1. The housing body 6 has a central recess 7, in which for example an LED chip can be disposed when the component is intended to be radiation-emitting.
The arrangement illustrated schematically in FIG. 1 is typical of a surface-mountable light-emitting semiconductor component, such as for example a miniature LED. In the example shown, the housing body 6, made for example of plastic, is already molded onto the leadframe 1. The leadframe 1 can be made for example of copper, possibly coated with another metal. During the subsequent manufacturing process of the component, a semiconductor chip is placed through the recess 7 onto one of the electrical lead elements 2 and is connected to the other electrical lead element 2 via a bonding wire. The opening 7 is then filled with light-transparent synthetic resin. This so-called premold technology, in which parts of the housing are molded onto a leadframe prior to the insertion of the semiconductor chip, is familiar to the specialist in optoelectronic components and thus will not be described further herein.
The electrical lead tabs 3 protrude axially from the oblong housing body 6. As the component manufacturing process continues, these lead tabs 3 are cut to a set length and bent. Such manufacturing processes, known as trim and form, require that the component be fixed in a retaining device. This purpose is served by the retention tabs 4. They extend just far enough into the housing body 6 so that they are able to transmit the necessary retention forces to the component when clamped in the retaining device and still can be removed from the housing body 6 by the application of slight pulling forces when no longer needed. Typical depths for which the retention tabs 4 extend into the housing body for this purpose are in the range of 5/100 mm. To be removable by the exertion of slight pulling forces, the retention tabs 4 must be separated adequately from the lead elements 2 in the leadframe 1.
According to the prior art, the separation of the retention tabs from the component is effected by means of a punching or etching process that results in a punched or etched gap 5. Given the prescribed dimensions of the housing body 6, this gap 5 requires that the lead elements 2 be tapered in the region of the retention tabs 4. This taper has the effect of making the leadframe 1 mechanically unstable, which can have adverse effects particularly during the steps of the manufacturing process in which no part of the housing body 6 has yet been molded onto the leadframe 1. Another consequence of tapering the lead elements 2 is that heat generated by the semiconductor chip during the operation of the component cannot be dissipated adequately via the lead tabs 3.